Precise rail tracking method for powering dual voltage integrated circuits

ABSTRACT

A rail tracking system and method for providing precise tracking of voltage levels to a dual supply voltage Integrated Circuit. A switch mode DC-DC voltage regulator is used to derive the lower of the two voltage levels from the higher level. The switch mode regulator employs a pulse width modulator (PWM) to derive the lower voltage level. A separate supply source is utilized to power the PWM and the timing of the supply voltage is such that the PWM has reached steady state before the higher voltage level is provided to the regulator.

FIELD OF THE INVENTION

[0001] This invention relates to integrated circuits having dual supplyvoltage requirements and, more particularly, to a system and method foraccurately controlling the dual supply voltage levels.

BACKGROUND OF THE INVENTION

[0002] Many large scale integrated circuits contain multiple componentsproviding different functionality and require two different supplyvoltage levels to operate. One such integrated circuit would include,for example, a core processor and input/output functions on the samesilicon substrate, but operating from two different voltage levels.During startup, steady state, shut down and under fault conditions, theinteraction between these voltages must meet strict requirements toensure proper operation and to prevent damage to the integrated circuit.The techniques used to ensure proper interaction of these voltage levelsall fall under the class of methods known as “Rail Tracking”.

[0003] In a dual supply voltage mode scenario, typically, the larger ofthe two voltage rails will supply the input/output function, and thesmaller of the two is used to power the core processor. The larger ofthe two voltage levels is supplied to the input/output functionality ofthe integrated circuit, and to a voltage regulator which derives thesecond or lower voltage level for use in powering the core processor.

[0004] The task of the voltage regulator consists of keeping the voltageon the output constant in a defined output range. One form of voltageregulator comprises a switch mode power supply. A switch mode powersupply usually comprises a pulse width modulator (PWM), a power switch,a rectifier and an output filter. The pulse width modulator controls thepower switch which converts an input voltage into pulsed DC voltage withvariable duty cycle which in effect maintains constant voltage on theoutput of the filter circuit. In conventional voltage regulator voltageto power the PWM circuit is derived from the regulator's input voltage.

[0005] Because the PWM circuit requires a finite period to achievesteady state conditions there is an initial period between the time thatthe voltage is supplied to the regulator input and the time in which theoutput is fixed at the second voltage level. During this time thevoltage difference between the input/output voltage and output corevoltage may exceed maximum allowable limits causing damage to theintegrated circuit.

[0006] A prior art method dealing with this problem is disclosed inPower Trends application note PT5000/6000 SIP Series (IntegratedSwitching Regulators DC-DC Converters). In this prior art solution, thevoltage regulator is bypassed by a number of series-connected diodes anda small resistor which are connected between the input/output voltagelevel and the core processor voltage. The series-connected diodes limitthe difference between the two voltage levels as will be discussed ingreater detail hereinafter.

[0007] There are shortcomings to this prior art method which render itunacceptable in certain circumstances. For example, as the steady statedifference between the input/output voltage and the core processorvoltage approaches the maximum allowable voltage difference, thetolerance on the diode voltage drop becomes critical. This tolerance isdifficult to control inasmuch as the voltage drop across the diodejunction is highly current and temperature dependent. Additionally, thetracking voltage difference can be set only with the resolution of eachsingle junction voltage drop which typically equals approximately 0.6volts or 0.3 volts for Schottky technology. Additionally, theseries-connected diodes bypassing the voltage regulator negate anyovercurrent protection provided by the voltage regulator. In addition,the diodes themselves can be easily damaged if the regulator fails asall of the current associated with the second voltage level will nowflow through the diodes.

SUMMARY OF THE INVENTION

[0008] Accordingly, there is a requirement for an improved rail trackingsystem and method for powering a dual voltage integrated circuit.

[0009] Therefore, in accordance with a first aspect of the presentinvention there is provided a rail tracking method for providing dualvoltages levels to first and second voltage rails on an integratedcircuit (IC) comprising: providing a first voltage to the first voltagerail; providing the first voltage to a voltage regulator havingconversion means to derive a second voltage for the second voltage rail;and providing a supply voltage to the conversion means whereby thesupply voltage is provided before the first voltage is provided to thevoltage regulator.

[0010] In accordance with a second aspect of the present invention thereis provided a system for providing rail tracking of dual voltage levelsto first and second voltage rails on an integrated circuit comprising:first voltage means to supply a first voltage level to the first voltagerail; a voltage regulator having means to receive the first voltagelevel; conversion means in the voltage regulator to derive a secondvoltage level for the second voltage rail from the first voltage level;and a supply voltage means to supply a supply voltage to the conversionmeans wherein the supply voltage is supplied to the conversion meansbefore the first voltage is supplied to the voltage regulator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will now be described in greater detail withreference to the attached drawings wherein:

[0012]FIG. 1 is a typical power circuit for dual voltage integratedcircuits;

[0013]FIG. 2 is a diode rail tracking circuit according to the priorart;

[0014]FIG. 3 is a linear series pass regulator rail tracking circuit;

[0015]FIG. 4 is a circuit diagram of a precise rail tracking methodologyaccording to the present invention; and

[0016]FIG. 5 is an example of a block diagram of a practicalimplementation of the tracking method according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1 illustrates a typical power circuit for a dual voltageintegrated circuit. Integrated circuit 12, which according to thepresent invention has a requirement for dual supply voltage levels,namely, a first voltage level for the input/output voltage (Vi/o) and asecond voltage level for the core processor (Vcore). As discussedpreviously Vi/o is at a higher voltage level than Vcore. An inputvoltage (Vin) is provided by a power supply (not shown) to Vi/o and tothe input of a voltage regulator 14. Voltage regulator 14 derives theVcore voltage level from Vin. Typically, voltage regulator 14 employsswitch mode topology to achieve the voltage conversion. The key elementof the switch mode topology is a pulse width modulator as discussedpreviously. The key aspect, however, is that the voltage regulatoroutput voltage reaches its steady state value, i.e. Vcore, only afterinitial stabilizing period has passed. As noted previously, during thisstabilizing period, the voltage difference between the input/outputvoltage and the output core voltage may exceed maximum allowable limitscausing damage to the integrated circuit.

[0018] The prior art solution to this problem is illustrated in FIG. 2wherein voltage regulator 14 is bypassed by series-connected diodes 16,18 and 20, and resistor 22. Typically, fuse 24, will protect the voltageregulator and downstream components.

[0019] According to the prior art method, the series-connected diodesare selected to provide a voltage drop across the voltage regulator suchthat the voltage difference between the input and at the core cannotexceed the maximum specified value. In FIG. 2, diodes 16, 18 and 20,limit the difference between Vi/o and Vcore. As discussed previously,there are shortcomings to the method illustrated in FIG. 2 which limitspractical implementations of the concept.

[0020] A rail tracking method developed by Newbridge NetworksCorporation uses a linear series pass regulator connected between theinput Vi/o and the output Vcore of the regulator. This method has provento be effective when high precision tracking (1.6 V maximum differencebetween Vi/o and Vcore, while the normal operation difference is only1.3 V) is required with high currents (6 to 20A). A circuit illustratingthis rail tracking method is shown in FIG. 3. Operational amplifier 30controls power transistor 38 which, in turn, provides Vcore during thetime it takes regulator 14 to startup. During the initial period,operational amplifier 30 is controlled by V reference 32 until theoutput across voltage divider 34/36 reaches the steady state value.Resistor 40 reduces power dissipation in transistor 38, and diode 42provides backward tracking during turnoff. An additional voltagemonitoring network, (not shown) is required to protect resistor 40 andtransistor 38 in the case of a failure of the regulator 14.

[0021] Although the rail tracking method shown in FIG. 3 is effective,multiple, real estate consuming, power components are required inaddition to the standard regulator.

[0022] Regulator 14 in the prior art and in the embodiment of FIG. 3typically employs a switch mode topology to achieve voltage conversion.The key element of this topology is the pulse width modulator (PWM) aspreviously discussed. The regulator will only produce the requiredoutput voltage after the PWM is operational. As discussed previously,the supply voltage for the PWM is derived from the regulator inputvoltage. Accordingly, there is an inherent delay between the voltagebeing applied to the regulator input and the PWM being operational. Thisaccounts for the delay between Vi/o and Vcore.

[0023] The preferred embodiment of the present invention is illustratedin the circuit diagram of FIG. 4. The basis of this invention relies onthe PWM power supply being connected prior to the regulator inputvoltage being applied. As a result, the regulator output voltage willtrack, with no delay, the regulator input voltage. This ensures truetracking between regulator input and output voltages which correspond tothe true tracking between Vi/o and Vcore.

[0024] As shown in FIG. 4, voltage regulator 14 includes pulse widthmodulator 50, which is supplied by supply voltage through input 52. Inaccordance with the basic concept of the invention, supply voltage isprovided through input 52 prior to Vi/o being supplied to the regulator.In this way, the pulse width modulator has reached steady statecondition before Vi/o is supplied and hence, the regulator outputvoltage (Vcore) will precisely track the regulator input voltage. Asshown in FIG. 4, the PWM supply voltage is an external voltage notrelated to the regulator input voltage.

[0025] The Schottky diode 54 provides backward tracking during turnoff.An additional voltage monitoring network (not shown) may be used toprotect the integrated circuit in case of failure of the regulator.

[0026] The tracking method provided by the embodiment of FIG. 4 offersthe following benefits over those previously described. First, thismethod provides precise rail tracking inasmuch as the pulse widthmodulator is fully operational before the regulator input voltage issupplied. Secondly, no additional power components are required whichresults in lower board space, lower cost of the design, and increasedreliability. Further, the regulator current limit is not bypassed as wasthe case in the prior art method.

[0027] The block diagram of FIG. 5 shows an example of a practicalapplication of the tracking method of the present invention asimplemented in a practical design.

[0028] Four isolated DC-DC converters are used to provide power to thesystem. Three of these converters (3.3 V and 2×2.5 V outputs) arestandard modules which operate in the input voltage range 36 to 75 V.The fourth one, the 5 V output, operates over a wide input range 18 to75 V, and is designed to start faster than the remaining three majorconverters.

[0029] The main function of the +5 V converter is to provide earlysupply voltage for the monitoring circuit 60 which, via On/Off pinscontrols the major converters. As shown, the early 5 V converter is notpart of the On/Off loop. The rail tracking in the system is requiredbetween the 3.3 V and 2.5 V rails and between the 3.3 V Vi/o and 2.0 VVcore for the dual power IC. Tracking between high current rails (3.3V/60A and 2.5 V/20A) has been provided using the linear series passregulator circuit as illustrated in FIG. 3. The tracking methodaccording to the preferred embodiment of FIG. 4 is used for providingtracking between the 3.3 V and 2.0 V rail for the dual power IC. The 3.3V to 2.0 V module is a non-isolated, DC-DC switch mode power supply.According to the preferred embodiment of the invention, the pin tosupply the supply voltage to the pulse width modulator is isolated inorder that voltage from the early 5 V supply can be connected directlyto the pulse width modulator.

[0030] Although example embodiments of the invention have been disclosedand illustrated, it will be apparent to one skilled in the art thatvariation to the basic concept can be implemented. Particularly theinput/output and core voltage levels and the DC-DC module type may bedifferent. It is to be understood, however, that such variations willfall within the true scope of the invention as defined by the appendedclaims.

1. A rail tracking method for providing dual voltage levels to first andsecond voltage rails on an integrated circuit (IC) comprising: providinga first voltage to said first voltage rail; providing said first voltageto a voltage regulator having conversion means to derive a secondvoltage for said second voltage rail; and providing a supply voltage tosaid conversion means whereby said supply voltage is provided beforesaid first voltage is provided to said voltage regulator.
 2. The methodaccording to claim 1 wherein said conversion means uses a pulse widthmodulator (PWM) to derive said second voltage from said first voltage.3. A system for providing rail tracking of dual voltage levels to firstand second voltage rails on an integrated circuit (IC) comprising: firstvoltage means to supply a first voltage level to said first voltagerail; a voltage regulator having means to receive said first voltagelevel; conversion means in said voltage regulator to derive a secondvoltage level for said second voltage rail from said first voltagelevel; and a supply voltage means to supply a supply voltage to saidconversion means wherein said supply voltage is supplied to saidconversion means before said first voltage is supplied to said voltageregulator.
 4. A system as defined in claim 4 wherein said conversionmeans includes a pulse width modulator (PWM).
 5. A system as defined inclaim 5 further including a backward tracking diode between said secondvoltage rail and said first voltage rail.
 6. A system as defined inclaim 5 wherein said first voltage level supplies input/output functionson said IC and said second voltage level powers a core processor on saidIC.
 7. A system as defined in claim 6 wherein said supply voltage issupplied by a DC-DC switch-mode power supply.
 8. A system for providingrail tracking to an Integrated Circuit (IC) within an ATM switch whereinsaid IC performs multiple functions requiring dual operating voltagelevels; said system comprising: a first DC-DC power supply to supply afirst voltage level to said IC; a voltage regulator circuit employing aswitch mode converter to receive said first voltage level and to derivetherefrom a second voltage level for said IC; and a second DC-DC powersupply to supply operating voltage to said switch mode converter;whereby said second DC-DC power supply supplies said operating voltageto said switch mode converter such that said converter is operationalbefore said first voltage level to derive said second voltage level issupplied to voltage regulator circuit.
 9. A system as defined in claim 8wherein said switch mode converter employs a pulse width modulator toderive said second voltage level.
 10. A system as defined in claim 8wherein said voltage regulator circuit has a separate connection forreceiving said supply voltage.